2. The Field of the Invention
The present invention relates to rendering images on display devices having pixels with separately controllable pixel sub-components. More specifically, the present invention relates to rendering characters on display devices using pixel sub-component precision, while preserving the same advance width of the characters that would be obtained by rendering the characters using full pixel precision processes.
3. The Prior State of the Art
With the advent of the information age, individuals worldwide spend substantial amounts of time viewing display devices. Flat panel display devices, such as liquid crystal display (LCD) devices, and cathode ray tube (CRT) display devices are two of the most common types of display devices used to render text and graphics. CRT display devices use scanning electron beams that are controlled in an analog manner to activate phosphor positioned on a screen. A pixel of a CRT display device that has been illuminated by the electron beams consists of a triad of dots, each of a different color. The dots included in a pixel are controlled together to generate what is perceived by the user as a single point or region of light having a selected color defined by a particular hue, saturation, and intensity. The individual dots in a pixel of a CRT display device are not separately controllable. CRT display devices have been widely used in combination with desktop personal computers, workstations, and in other computing environments in which portability is not an important consideration.
LCD display devices, in contrast, have pixels consisting of multiple separately controllable pixel sub-components. Typical LCD devices have pixels with three pixel sub-components, which usually have the colors red, green, and blue. Each pixel sub-component of the pixels of an LCD device is a discrete light emitting device that can be individually controlled using analog or digital processes, depending on the nature of the LCD device. LCD devices have become widely used in portable or laptop computers due to their size, weight, and relatively low power requirements. Over the years, however, LCD devices have begun to be more common in other computing environments, and have become more widely used with non-portable personal computers.
Conventional image data and image rendering processes were developed and optimized to display images on CRT display devices. The smallest unit on a CRT display device that is separately controllable is a pixel; the three dots included in each pixel are controlled together to generate the desired color. Conventional image processing techniques map samples of image data to entire pixels, with the three dots included in the pixel together representing a single portion of the image. In other words, each pixel of a CRT display device corresponds to or represents a single region of the image data.
The image data and image rendering processes used with LCD devices are those that have been originally developed in view of the CRT, three-dot pixel model. Thus conventional image rendering processes used with LCD devices do not take advantage of the separately controllable nature of pixel sub-components of LCD pixels, but instead generate together the luminous intensity values to be applied to the three pixel sub-components in order to yield the desired color. Using these conventional processes, each three-part pixel represents a single region of the image data.
It has been observed that the eyestrain and other reading difficulties that have been frequently experienced by computer users diminish as the resolution of display devices and the characters displayed thereon improve. The problem of poor resolution is particularly evident in flat pane) display devices, such as LCDs, which may have resolutions 72 or 96 dots (i.e., pixels) per inch (dpi), which is lower than most CRT display devices. Such display resolutions are far lower than the 600 dpi resolution supported by most printers. Even higher resolutions are found in most commercially printed text such as books and magazines. The relatively few pixels in LCD devices are not enough to draw smooth character shapes, especially at common text sizes of 10, 12, and 14 point type. At such common text rendering sizes, portions of the text appear more prominent and coarse on the display device than when displayed on CRT display devices or printed.
In view of the foregoing problems experienced in the art, there is a need for techniques of improving the resolution of images displayed on LCD display devices. Because any such improved rendering process would be likely to be used to display electronic documents that were originally created with legacy rendering processes, it would be important to maintain the formatting and overall document size when such existing electronic documents are displayed.
The present invention relates to image data processing and image rendering techniques whereby images are displayed on display devices having pixels with separately controllable pixel sub-components. Spatially different sets of one or more samples of image data are mapped to individual pixel sub-components rather than to full pixels. The resulting displayed image has at least some features that are rendered with sub-pixel precision. According to the invention, the advance width of text characters as displayed using the sub-pixel precision rendering processes are selected to be compatible with the advance width of the characters if they were to be instead rendered with a conventional full pixel precision rendering process. In other words, the characters are displayed according to the invention with sub-pixel precision, while the advance width and the corresponding size and spacing of the characters and the overall formatting of the document remains the same compared to documents and characters rendered using conventional full pixel precision processes.
The invention is particularly suited for use with LCD display devices or other display devices having pixels with a plurality of pixel sub-components of different colors. For example, the LCD display device may have pixels with red, green, and blue pixel sub-components arranged on the display device to form either vertical or horizontal stripes of same-colored pixel sub-components.
According to one implementation of the invention, image data representing one or more text characters is processed using a scaling operation, a hinting operation, and a scan conversion operation. After the image data is scaled in the scaling operation, selected points of the scaled image data are adjusted to a grid corresponding to the pixels of the displays device in the hinting operation. As part of the hinting operation, the left and right side bearing points of the character are repositioned to locations on the grid that correspond to full pixel boundaries in a way such that the resulting advance width is compatible with, or equal to, the advance width that would be generated using a full pixel precision rendering process.
During the hinting operation, the left edges of the stems of the character are adjusted to full pixel boundaries. Other points of the image data are either directly positioned or adjusted on the grid, or are interpolated based on previously positioned points. The hinting process involves identifying the selected points of the character that are to be repositioned with respect to the grid. Accordingly, the topology of the character is analyzed during runtime to identify these selected points. Because a direct and full analysis of character topology can consume significant processing time and resources, an indirect or partial topological analysis can be performed.
The hinted image data with the compatible advance widths is then processed according to the scan conversion process. In order to display the character with sub-pixel precision, spatially different sets of one or more samples of the image data are mapped to individual pixel sub-components of the pixels, rather than to entire pixels.
The invention can be used in a variety of computing and software environments to yield displayed characters and other images having improved resolution. For instance, the preservation of compatible advance widths can have significant benefits when used with electronic documents that are originally generated or are later displayed in connection with conventional full pixel precision rendering processes. Because the rendering processes of the invention result in compatible advance widths, documents generated or displayed according to the invention typically have the same general formatting if they are also displayed or generated using conventional rendering processes. Thus, the invention provides desirable interoperability between the sub-pixel precision rendering processes disclosed herein and conventional rendering processes.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.